Organic light emitting devices having carrier transporting layers comprising metal complexes

ABSTRACT

Light emitting devices having charge transporting layers comprising one or more metal complexes are provided. More particularly, devices include hole transporting layers comprising at least one organometallic complex are disclosed. The present devices can further comprise an electron blocking layer for improved efficiency.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/226,861, filed Aug. 23, 2002, now U.S. Pat. No. 7,078,113 whichclaims priority to Provisional Patent Application Ser. No. 60/315,527,filed Aug. 29, 2001, and Application No. 60/317,541, filed Sep. 5, 2001,which are incorporated herein by reference in their entirety. Thisapplication is related to copending Provisional Application No.60/317,540, filed Sep. 5, 2001, which is incorporated herein byreference in its entirety.

GOVERNMENT RIGHTS

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Contract No.F33615-94-1-1414 awarded by DARPA.

RESEARCH AGREEMENTS

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a jointuniversity-corporation research agreement: Princeton University, TheUniversity of Southern California, and Universal Display Corporation.The agreement was in effect on and before the date the claimed inventionwas made, and the claimed invention was made as a result of activitiesundertaken within the scope of the agreement.

FIELD OF THE INVENTION

The present invention is directed to light emitting devicesincorporating metal complexes for improved efficiency and stability.

BACKGROUND OF THE INVENTION

Electronic display currently is a primary means for rapid delivery ofinformation. Television sets, computer monitors, instrument displaypanels, calculators, printers, wireless phones, handheld computers, etc.aptly illustrate the speed, versatility, and interactivity that arecharacteristic of this medium. Of the known electronic displaytechnologies, organic light emitting devices (OLEDs) are of considerableinterest for their potential role in the development of full color,flat-panel display systems that may render obsolete the bulky cathoderay tubes still currently used in many television sets and computermonitors.

Generally, OLEDs are comprised of several organic layers in which atleast one of the layers can be made to electroluminesce by applying avoltage across the device (see, e.g., Tang, et al., Appl. Phys. Lett.1987, 51, 913, and Burroughes, et al., Nature, 1990, 347, 359). When avoltage is applied across a device, the cathode effectively reduces theadjacent organic layers (i.e., injects electrons) whereas the anodeeffectively oxidizes the adjacent organic layers (i.e., injects holes).Holes and electrons migrate across the device toward their respectiveoppositely charged electrodes. When a hole and electron meet on the samemolecule, recombination is said to occur and an exciton is formed.Recombination of the hole and electron is preferably accompanied byradiative emission, thereby producing electroluminescence.

Depending on the spin states of the hole and electron, the exciton whichresults from hole and electron recombination can have either a tripletor singlet spin state. Luminescence from a singlet exciton results influorescence whereas luminescence from a triplet exciton results inphosphorescence. Statistically, for organic materials typically used inOLEDs, about one quarter of the excitons are singlets and the remainingthree quarters are triplets (see, e.g., Baldo, et al., Phys. Rev. B,1999, 60, 14422). Until the discovery that there were certainphosphorescent materials that could be used to fabricate practicalelectro-phosphorescent OLEDs having a theoretical quantum efficiency ofup to 100% (i.e., harvesting all of both triplets and singlets), themost efficient OLEDs were typically based on materials that fluoresced.These materials fluoresced with a maximum theoretical quantum efficiencyof only 25% (where quantum efficiency of an OLED refers to theefficiency with which holes and electrons recombine to produceluminescence), since the triplets to ground state transition is formallya spin forbidden process. Electro-phosphorescent OLEDs have now beenshow to have superior overall device efficiencies as compared withelectro-fluorescent OLEDs (see, e.g., Baldo, et al., Nature, 1998, 395,151 and Baldo, e.g., Appl. Phys. Lett. 1999, 75(3), 4).

Typically, OLEDs contain several thin organic layers between a holeinjecting anode layer, comprising an oxide material such as indium-tinoxide (ITO), Zn—In—SnO₂, SbO₂, or the like, and an electron injectingcathode layer, comprising a metal layer such as Mg, Mg:Ag, or LiF:Al. Anorganic layer residing in proximity to the anode layer is usuallyreferred to as the “hole transporting layer” (HTL) due to its propensityfor conducting positive charge (i.e., holes). Various compounds havebeen used as HTL materials. The most common materials consist of varioustriaryl amines which show high hole mobilities. Similarly, the organiclayer residing in proximity to the cathode layer is referred to as the“electron transporting layer” (ETL) due to its propensity to conductnegative charge (i.e., electrons). There is somewhat more variety in theETL materials used in OLEDs as compared with for the HTL. A common ETLmaterial is aluminum tris(8-hydroxyquinolate) (Alq₃). Collectively, theETL and HTL are often referred to as carrier layers. In some cases, anadditional a layer may be present for enhancing hole or electroninjection from the electrodes into the HTL or ETL, respectively. Theselayers are often referred to as hole injecting layers (HILs) or electroninjecting layer (EIL). The HIL may be comprised of a small molecule suchas 4,4′,4″-tris(30methylphenylphenylamino)triphenylamine (MTDATA) orpolymeric material such as poly(3,4-ethylenedioxythiophene) (PEDOT). TheEIL may be comprised of a small molecule material such as, e.g., copperphthalocyanine (CuPc). Many OLEDs further comprise an emissive layer(EL), or alternatively termed, luminescent layer, situated between theETL and HTL, where electroluminescence occurs. Doping of the luminescentlayer with various luminescent materials has allowed fabrication ofOLEDs having a wide variety of colors.

In addition to the electrodes, carrier layers, and luminescent layer,OLEDs have also been constructed with one or more blocking layers tohelp maximize efficiency. These layers serve to block the migration ofholes, electrons, and/or excitons from entering inactive regions of thedevice. For example, a blocking layer that confines holes to theluminescent layer effectively increases the probability that holes willresult in a photoemissive event. Hole blocking layers desirably have adeep (i.e., low) HOMO energy level (characteristic of materials that aredifficult to oxidize) and conversely, electron blocking materialsgenerally have a high LUMO energy level. Exciton blocking materials havealso been shown to increase device efficiencies. Triplet excitons, whichare relatively long-lived, are capable of migrating about 1500 to 2000Å, which is sometimes greater than the entire width of the device. Anexciton blocking layer, comprising materials that are characterized by awide band gap, can serve to block loss of excitons to non-emissiveregions of the device.

In seeking greater efficiencies, devices have been experimentallycreated with layers containing light emitting metal complexes.Functional OLEDs having emissive layers oftris(2,2′-bipyridine)ruthenium(II) complexes or polymer derivativesthereof have been reported in Gao, et al., J. Am. Chem. Soc., 2000, 122,7426, Wu, et al., J. Am. Chem. Soc. 1999, 121, 4883, Lyons, et al., J.Am. Chem. Soc. 1998, 120, 12100, Elliot, et al., J. Am. Chem. Soc. 1998,120, 6781, and Maness, et al., J. Am. Chem. Soc. 1997, 119, 3987.Iridium-based and other metal-containing emitters have been reported in,e.g., Baldo, et al., Nature, 1998, 395, 151; Baldo, et al., Appl. Phys.Lett., 1999, 75, 4; Adachi, et al., Appl. Phys. Lett., 2000, 77, 904;Adachi, et al., Appl. Phys. Lett., 2001, 78, 1622; Adachi, et al., Bull.Am. Phys. Soc. 2001, 46, 863, Wang, et al., Appl. Phys. Lett., 2001, 79,449, and U.S. Pat. Nos. 6,030,715; 6,045,930; and 6,048,630. Emissivelayers containing (5-hydroxy)quinoxaline metal complexes as hostmaterial has also been described in U.S. Pat. No. 5,861,219. Efficientmulticolor devices and displays are also described in U.S. Pat. No.5,294,870 and International Application Publication No. WO 98/06242.

A metal-containing blocking layer has also been reported. Specifically,(1,1′-biphenyl)-4-olato)bis(2-methyl-8-quinolinolato N1,O8)aluminum(BAlq) has been used as a blocking layer in the OLEDs reported inWatanabe, et al. “Optimization of driving lifetime durability in organicLED devices using Ir complex,” in Organic Light Emitting Materials andDevices IV, Kafafi, ed. Proceedings of SPIE Vol. 4105, p. 175 (2001).

Although OLEDs promise new technologies in electronic display, theyoften suffer from degradation, short life spans, and loss of efficiencyover time. The organic layers can be irreversibly damaged by sustainedexposure to the high temperatures typically encountered in devices.Multiple oxidation and reduction events can also cause damage to theorganic layers. Consequently, there is a need for the development of newmaterials for the fabrication of OLEDs. Compounds that are stable toboth oxidation and reduction, have high T_(g) values, and readily formglassy thin films are desirable. The invention described hereinbelowhelps fulfill these and other needs.

SUMMARY OF THE INVENTION

The present invention provides light emitting devices comprising a holetransporting layer that includes at least one metal complex. In someembodiments, the hole transporting layer consists essentially of saidmetal complex or complexes. In further embodiments, the holetransporting layer comprises an organic matrix doped with said metalcomplex or complexes.

In some embodiments, the metal complex is coordinatively saturated,preferably wherein metal complex has a coordination number of four orsix. In some embodiments, the metal the said metal complex is atransition metal, which can be a first row, second row or third rowtransition metal. In some embodiments, the metal of said metal complexis Fe, Co, Ru, Pd, Os or Ir, or any subcombination thereof.

In some embodiments of the light emitting devices of the invention, atleast one metal complex has one of the formulas I or II:

wherein:

M′ and M′″ are each, independently, a metal atom;

R₁₀, R₁₃, R₂₀, and R₂₁, are each, independently, N or C;

R₁₁ and R₁₂ are each, independently, N or C;

Ring systems A, B, G, K and L are each independently a mono-, di- ortricyclic fused aliphatic or aromatic ring system optionally containingup to 5 hetero atoms;

Z is C₁-C₆ alkyl, C₂-C₈ mono- or poly alkenyl, C₂-C₈ mono- or polyalkynyl, or a bond; and

Q is BH, N, or CH.

In some embodiments where the metal complex has the formula I, Ringsystem A and Ring system B are each monocyclic. In further embodimentswhere the metal complex has the formula I, Ring system A is a fivemembered heteroaryl monocyclic ring and Ring system B is a six memberedaryl or heteroaryl monocyclic ring.

In some further embodiments, R₁₀ and R₁₁ are N, and R₁₃ is CH. Infurther embodiments, Ring A forms pyrazole. In still furtherembodiments, Ring B forms phenyl.

In some embodiments of the light emitting devices of the invention, themetal of at least one of the metal complexes is a d⁰, d¹, d², d³, d⁴,d⁵, or d⁶ metal. In some embodiments where the metal complex has theformula I, M′ is a transition metal. In further embodiments, M′ is Fe,Co, Ru, Pd, Os, or Ir. In further embodiments, M′ is Fe or Co. Infurther embodiments, M′ Fe, and in still further embodiments, M′ is Co.

In some embodiments of the light emitting devices of the invention wherethe metal complex has the formula I, the metal complex is Co(ppz)₃.

In some embodiments of the light emitting devices of the invention, atleast one metal complex has the formula II. In some embodiments of thelight emitting devices of the invention where at least one metal complexhas the formula II, Ring systems G, K and L are each 5 or 6 membermonocyclic rings. In further embodiments, the Ring systems G, K, and Lare each 5-membered heteroaryl monocyclic rings. In still furtherembodiments, R₂₁ and R₂₂ are each N.

In further embodiments of the light emitting devices of the inventionwhere at least one metal complex has the formula II, Ring systems G, Kand L each form pyrazole. In further embodiments, M′″ is a d^(5/6) ord^(2/3) metal.

In some embodiments of the light emitting devices of the invention whereat least one metal complex has the formula II, M′″ is a transitionmetal. In further embodiments, M′″ is Fe, Co, Ru, Pd, Os, or Ir. Infurther embodiments, M′″ is Fe or Co. In further embodiments, M′″ is Fe.

In some embodiments, the metal complex is FeTp′₂.

In some embodiments of the light emitting devices of the invention, thelight emitting device further comprises an electron blocking layer,which can be include an organic electron blocking material, a metalcomplex, or both. In some embodiments, the organic electron blockingmaterial is selected from triarylamines or benzidenes. In someembodiments, the electron blocking layer consists essentially of themetal complex. In further embodiments, the electron blocking layercomprises a matrix doped with said metal complex.

In some embodiments, the electron blocking layer has a HOMO energy levelclose to the HOMO energy level of said hole transporting layer. Infurther embodiments, the electron blocking layer has a HOMO energy levelhigher than the HOMO energy level of said hole transporting layer.

In some embodiments, the metal complex of the electron blocking layercomprises a metal selected from Ga, In, Sn, or a group 8, 9, or 10transition metal. In some embodiments, the metal complex of saidelectron blocking layer comprises Ga. In further embodiments, the metalcomplex of the electron blocking layer comprises a multidentate ligand.In some embodiments, the multidentate ligand has a bridging atomselected from N and P. In further embodiments, the multidentate ligandhas a mesityl bridge moiety. In further embodiments, the multidentateligand comprises up to three mono-, bi- or tricyclic heteroaromaticmoieties.

In some embodiments, the of the light emitting devices of the invention,the electron blocking layer comprises a compound having the formula III:

wherein:

M is a metal atom;

X is N or CX′ where X′ is H, C₁-C₂₀ alkyl, C₂-C₄₀ mono- or poly alkenyl,C₂-C₄₀ mono- or poly alkynyl, C₃-C₈ cycloalkyl, aryl, heteroaryl,aralkyl, heteroaralkyl, or halo;

A is CH, CX′, N, P, P(═O), aryl or heteroaryl;

each R¹ and R² is, independently, H, C₁-C₂₀ alkyl, C₂-C₄₀ alkenyl,C₂-C₄₀ alkynyl, C₃-C₈ cycloalkyl, aryl, aralkyl, or halo; or

R¹ and R², together with the carbon atoms to which they are attached,link to form a fused C₃-C₈ cycloalkyl or aryl group;

R³ is H, C₁-C₂₀ alkyl, C₂-C₄₀ alkenyl, C₂-C₄₀ alkynyl, C₃-C₈ cycloalkyl,ary aralkyl, or halo; and

n is 1 to 5.

In some embodiments where the electron blocking layer has the formulaIII, M is a trivalent metal atom, X is CH or N, A is N or 1,3,5-phenyl,each R¹ and R² are H, or R¹ and R², together with the carbon atoms towhich they are attached, link to form a phenyl group, R³ is H; and n is1 or 2. In further embodiments where the electron blocking layer has theformula III, M is Ga.

In further embodiments, the electron blocking layer comprises Ga(pma)₃.

In some embodiments of the light emitting devices of the invention, themetal complex of said hole transporting layer is Co(ppz)₃. In furtherembodiments of the light emitting devices of the invention, the metalcomplex of said hole transporting layer is FeTp′₂.

In further embodiments of the light emitting devices of the invention,the electron blocking layer comprises Ga(pma)₃ and the metal complex ofsaid hole transporting layer is Co(ppz)₃.

In further embodiments of the light emitting devices of the invention,the electron blocking layer comprises Ga(pma)₃ and the metal complex ofsaid hole transporting layer is FeTp′₂.

The present invention also provides light emitting devices comprisingthe substructure HTL/EL or HTL/EBL/EL; wherein each of said EL, HTL, andEBL comprise at least one metal complex.

Also provided by the present invention are light emitting devicescomprising the substructure HTL/EL or HTL/EBL/EL; wherein none of saidEL, HTL, or EBL is comprised solely of organic molecules.

The present invention further provides light emitting devices having aplurality of layers, the devices being devoid of a layer that iscomposed solely of organic molecules. In some embodiments, each of saidlayers contains at least one metal complex.

The present invention also provides light emitting devices comprising ahole transporting layer, an emissive layer, and a blocking layer;

said hole transporting layer having a first HOMO energy, wherein saidhole transporting layer comprises at least one metal complex;

said emissive layer comprising at least one material capable oftransporting electrons, said material having a second HOMO energy; and

said blocking layer comprising a material having a HOMO energy that isbetween said first and second HOMO energies.

In some embodiments, the blocking layer resides between said holetransporting layer and said emissive layer. In further embodiments, theblocking layer comprises an organic electron blocking material, whichcan be, but is not necessarily, selected from triarylamines orbenzidenes. In still further embodiments, electron blocking layercomprises a metal complex.

The present invention also provides methods of facilitating holetransport in a light emitting device, said light emitting devicecomprising a hole transporting layer and an emissive layer;

said hole transporting layer comprising at least one metal complex andhaving a first HOMO energy;

said emissive layer comprising at least one material capable oftransporting electrons, said material having a second HOMO energy higherthan said HOMO energy of said hole transporting layer;

said method comprising the step of placing a blocking layer between saidhole transporting layer and said emissive layer, wherein said blockinglayer comprises a material having a HOMO energy level that isintermediate between said first and second HOMO energies.

In some embodiments of the foregoing methods, the metal complex of saidhole transporting layer is a complex formula I or II, as describedabove. In some embodiments, the metal complex of the hole transportinglayer is Co(ppz)₃ or FeTp′₂. In further embodiments, the blocking layercomprises at least one metal complex. In some embodiments, the metalcomplex of said blocking layer is a compound having the formula III asdescribed above. In further embodiments, the metal complex of theblocking layer is Ga(pma)₃. In further embodiments, the holetransporting layer comprises Co(ppz)₃ or FeTp′₂, and said barrier layercomprises Ga(pma)₃.

The present invention also provides methods of fabricating a lightemitting device, said method comprising placing a hole transportinglayer in electrical contact with an emissive layer, wherein said holetransporting layer comprises a compound of formulas I or II as describedabove. In some embodiments, the light emitting device further comprisesan electron blocking layer. In further embodiments, the electronblocking layer comprises a compound having the formula III as describedabove. In further embodiments, the electron blocking layer comprisesGa(pma)₃. In further embodiments, the compound is a compound of formulaI, and is Co(ppz)₃ or FeTp′₂. In further embodiments, the compound ifCo(ppz)₃.

The present invention also provides methods of transporting holes in ahole transporting layer of a light emitting device, wherein said holetransporting layer comprises at least one metal complex, said methodcomprising applying a voltage across said device.

Also provided by the present invention are pixels and displayscomprising the devices described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 compares plots of quantum efficiency v. current density fordevices of the structure ITO/NPD(500 Å)/Alq₃(600 Å)/Mg:Ag andITO/Co(ppz)₃(500 Å)/Alq₃(600 Å)/Mg:Ag.

FIG. 2 compares plots of current density v. voltage for devices of thestructure ITO/NPD(500 Å)/Alq₃(600 Å)/Mg:Ag and ITO/Co(ppz)₃(500Å)/Alq₃(600 Å)/Mg:Ag.

FIG. 3 compares plots of current density v. voltage for devices of thestructure ITO/NPD(500 Å)/Alq₃(600 Å)/Mg:Ag and ITO/Co(ppz)₃(500Å)/Alq₃(600 Å)/Mg:Ag.

FIG. 4A-4C compares plots of quantum efficiency v. current density fordevices comprising Co(ppz)₃.

FIG. 5 compares plots of current density v. voltage for devicescomprising Co(ppz)₃.

FIGS. 6A-6B show electronic spectra for Ga(pma)₃.

FIG. 7 shows a current density v. voltage plot for a device having thestructure ITO/Co(ppz)₃(400 Å)/Ga(pma)₃(100 Å)/Alq₃(500 Å)/Mg:Ag(1000Å)/Ag.

FIG. 8 shows a luminance v. voltage plot for devices of the structureITO/Co(ppz)₃(400 Å)/Ga(pma)₃(100 Å)/Alq₃(500 Å)/Mg:Ag(1000 Å)/Ag.

FIG. 9 shows an external quantum efficiency v. voltage plot for devicesof the structure ITO/Co(ppz)₃(400 Å)/Ga(pma)₃(100 Å)/Alq₃(500Å)/Mg:Ag(1000 Å)/Ag.

FIG. 10 shows an external quantum efficiency v. current density plot fordevices of the structure ITO/Co(ppz)₃(400 Å)/Ga(pma)₃(100 Å)/Alq₃(500Å)/Mg:Ag(1000 Å)/Ag.

FIGS. 11A-11B show current density v. voltage and brightness v. voltageplots for devices having the structure ITO/HTL(500 Å)/CBP:Irppy(6%)(200Å)/BCP(150 Å)Alq₃(200 Å)/LiF/Al.

FIGS. 12A-12B show quantum efficiency v. current density and emissionspectrum plots for devices having the structure ITO/HTL(500Å)/CBP:Irppy(6%)(200 Å)/BCP(150 Å)Alq₃(200 Å)/LiF/Al.

FIG. 13 shows a plot of current v. voltage for devices of the structureITO/Co(ppz)₃(400 Å)/NPD(100 Å)/Alq₃(500 Å)/Mg:Ag(1000 Å)/Ag(400 Å).

FIG. 14 shows a plot of brightness v. voltage for devices of thestructure ITO/Co(ppz)₃(400 Å)/NPD(100 Å)/Alq₃(500 Å)/Mg:Ag(1000Å)/Ag(400 Å).

FIGS. 15A-15B show a plots of quantum efficiency v. voltage and quantumefficiency v. current density for devices of the structureITO/Co(ppz)₃(400 Å)/NPD(100 Å)/Alq₃(500 Å)/Mg:Ag(1000 Å)/Ag(400 Å).

FIG. 16 illustrates some hole transporting materials used in OLEDS.Triarylamine derivatives have proved to be excellent hole transportingmaterials for OLEDS. The hole mobilities in these materials are veryhigh, due to the high degree of intermolecular overlap and the planarnature of both the neutral and cationic forms of the molecules. The factthat both the neutral and cationic (hole) states for triarylamines leadsto low reorganization energies and thus low barriers to electrontransfer.

FIG. 17 illustrates some metal complexes suitable in layers of thedevices of the present invention. Several devices have been madeincorporating the organometallic complexes depicted in FIG. 17, whichincorporate metals that have the ability to assume a variety ofoxidation states and low kinetic barriers inherent in someorganometallic self-exchange reactions.

FIG. 18 describes aspects of carrier migration. Electron self-exchangereactions, where A⁺+A→A+A⁺ leads to carrier migration. Before electrontransfer from one center to another can take place, the system mustadjust so that the energy of the system is unaltered during electrontransfer. At the instant of electron transfer, the nuclei remainstationary, with no energy change in the system. The amount ofdistortion reciuired to reach a common state is inversely related to therate of electron transfer. Since t_(2g) is a nonbonding orbital, theelectrons into and out of the orbital involves small M-L changes.Electron hopping between the two oxidation states may take place withease when the two oxidation states have essentially the same structures.

FIG. 19 shows chemical syntheses of metal complexes suitable in devicesof the present invention, where X=H, CH₃, OCH₃.

FIG. 20A shows absorption spectra of several Co compounds suitable indevices of the present invention. The intense peak below 300 nm isassigned to the ligand-centered π-π* transition. The broad structurelessband around 340 nm is assigned to a metal-to-ligand charge transfer(MLCT) transition. The broad shoulder in the low 400 nm rangecorresponds to the d-d transition, characteristic of octahedral Co³⁺complexes. FIG. 20B shows the absorption spectrum for PPZ.

FIG. 21 shows cyclic voltammograms for Co complexes suitable in devicesof the present invention for the reaction Co(□)PPZ→Co(IV)PPZ⁺. Theelectron-rich phenylpyrazole ligand stabilizes the Co(□) speciesproduced electrochemically. UPS Data (HOMO) for the voltammograms is afollows:

CoPPZ: 5.37 eV

Co(MPPZ): 5.38 eV

NPD: 5.51 eV

FIG. 22 illustrates properties of devices comprising Co compounds,having the device structure ITO/Co(xppz)(250 Å)/Alq(500)/Mg:Ag(1000Å)/Ag(400 Å), where X=H for the filled squares, X is OCH₃ for the filledcircles, and X is CH₃ for the filled triangles.

FIG. 23 illustrates properties of devices comprising Co(ppz)₃ and an NPDelectron blocking layer.

FIG. 24 illustrates photophysical properties of Ga(pma)₃. The compound,Ga(pma)₃ is energetically suitable as a hole transporting/electronblocking layer between Coppz and Alq because the HOMO energy level is at5.74 eV (calculated using ultraviolet emission spectroscopy) and theLUMO energy level is 2.34 eV. A wide energy gap of about 3.4 eV can beobtained from the absorption and emission spectra. The hole transportingproperties are located at the tertiary N atom and not at the metalcenter.

FIG. 25 illustrates properties of devices devoid of a purely organiclayer.

FIG. 26 illustrates properties of devices devoid of a purely organiclayer.

FIG. 27 compares properties of devices comprising NPD with properties ofdevices comprising Co and Ga metal complexes instead of NPD.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As used herein, the terms “low” and “deep,” in reference to molecularorbital energies, are used interchangeably. Lower and deeper generallydescribe molecular orbitals residing at a lower, or more stable, energylevel. Ionization of electrons from deeper orbitals requires more energythan ionization of electrons in shallower, or higher, orbitals. Thus,although the deeper orbitals are said to be lower, they are oftenreferred to numerically by higher numbers. For example, a molecularorbital residing at 5.5 eV is lower (deeper) than a molecular orbitalresiding at 2.5 eV. Similarly, the terms “shallow” and “high” inreference to orbital energy levels, refer to orbitals at less stableenergies. These terms are well known to those skilled in the art.

As used herein, the term “adjacent,” in reference to layers of lightemitting devices, refers to layers having contacting sides. For example,a first layer and a second layer that are adjacent to one anotherdescribe, for example, contacting layers where one side of one layer isin contact with one side of the other layer.

As used herein, the term “gap” or “band-gap” generally refers to anenergy difference, such as, for example, between a HOMO and a LUMO. A“wider gap” refers to an energy difference that is greater than for a“narrower gap” or “smaller gap.” A “carrier gap” refers to the energydifference between the HOMO and LUMO of a carrier.

The present invention is directed to, inter alia, light emitting devicescomprising one or more layers that in turn comprise at least one metalcomplex. Devices, as such, may have higher efficiencies and higherstability as compared with devices having traditional organic blockinglayers.

The light emitting devices of the present invention are typicallylayered structures that electroluminesce when a voltage is appliedacross the device. Typical devices are structured so that one or morelayers are sandwiched between a hole injecting anode layer and anelectron injecting cathode layer. The sandwiched layers have two sides,one facing the anode and the other facing the cathode. These sides arereferred to as the anode side and the cathode side, respectively. Layersare generally deposited on a substrate, such as glass, on which eitherthe anode layer or the cathode layer may reside. In some embodiments,the anode layer is in contact with the substrate. In many cases, forexample when the substrate comprises a conductive or semi-conductivematerial, an insulating material can be inserted between the electrodelayer and the substrate. Typical substrate materials, that may be rigid,flexible, transparent, or opaque, include glass, polymers, quartz,sapphire, and the like.

Hole transporting layers are placed adjacent to the anode layer tofacilitate the transport of holes. In some embodiments, a hole injectinglayer for enhancing hole injection, sometimes referred to as a holeinjecting enhancement layer, may be placed adjacent to the anode,between the anode and the HTL. Materials suitable for the HTL includeany material that is known by one skilled in the art to function assuch. Suitable materials are typically easy to oxidize and includetriaryl amines such as N,N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′biphenyl-4,4′diamine (TPD),4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD),4,4′-bis[N-(2-naphthyl)-N-phenyl-amino]biphenyl (β-NPD), and the like.Metal complexes may also be used in HTLs. Some suitable metal complexesare described, for example, in Application Ser. No. 60/283,814, filedApr. 13, 2001, which is incorporated herein by reference in itsentirety. Similarly, ETLs are situated adjacent to the cathode layer tofacilitate transport of electrons. An electron injecting enhancementlayer can optionally be placed adjacent to ETL or cathode layer.Materials suitable for the ETL include any materials known in the art tofunction as such. Typical ETL materials are relatively easy to reduceand can include, for example, aluminum tris(8-hydroxyquinolate) (Alq₃),carbazoles, oxadiazoles, triazoles, thiophene, oligothiophene, and thelike. HTL and ETL carrier layers can have thicknesses ranging from about100 to about 1000 Å. Since it is typically the site of exciton formationand luminescence, the EL layer is preferably somewhere between the HTLand ETL. The EL can optionally be in contact with one or both of the HTLand ETL or may be flanked by one or more blocking layers. EL materialscan include, for example dye-doped Alq₃ and the like. In someembodiments, neat (un-doped) films of luminescent material may be usedas the emissive layer. Furthermore, layers can serve dual functions. Forexample, an ETL or HTL can also function as an EL.

In some embodiments according to the present invention, devices compriseat least one charge transport layer (i.e., carrier layer), such as, forexample, a HTL, ETL, hole injecting layer, or electron injecting layer,that comprises at least one metal complex. The carrier layer can be athin film consisting essentially of metal complex or an organic matrixdoped with metal complex. In some preferred embodiments, devices of thepresent invention comprise a hole blocking layer that includes at leastone metal complex. Accordingly, in some embodiments, the metal complexesof the HTLs can be selected such that their HOMO energy levels willsupport hole transport. Thus, in some embodiments, it is preferable toselect metal complexes having HOMO energy levels in the range of fromabout 7 to 4.5 eV and LUMO energy levels in the range of from about 2 toabout 4 eV.

Metal complexes that are suitable for use in devices as a chargetransport layer can be selected on the basis of properties thatfacilitate this function. For example, a hole transporting materialtypically undergoes one electron oxidation. Thus, metal complexes thatare stable to one electron oxidation processes can be suitable as holetransporters. Similarly, metal complexes that are stable to one electronreductions can be suitable as electron transporting materials. Stableoxidation and reduction processes can be identified by electrochemicalmethods such as cyclic voltammetry, which is further discussed below.Another consideration is charge mobility. For example, materials havinghigh hole mobilities can generally function as good hole transporters.Charge mobility often corresponds with low reorganizational energies forredox processes. Thus, for example, metal complexes showing littlestructural differences when oxidized or reduced typically have arelatively small energy barrier (reorganizational energy) associatedwith oxidation or reduction. Certain metal properties such as electronconfiguration can affect reorganizational barriers, as is well known inthe art and further discussed below. Additionally, certain ligandproperties, such as denticity, can affect reorganizational barriers ofredox events in metal complexes, the details of which are also furtherdiscussed below.

In some embodiments, it is desirable that one or more layers of thedevice comprise one or more dopants. Emissive dopants (i.e.,photoemitting molecules, emitters) can be included in at least onelayer, such as for example the EL, for improved efficiency and colortunability. Doped layers usually comprise a majority of host materialand minority of dopant. Host material (also referred to as matrix)typically transfers excitons through a non-radiative process to theemissive dopant material, which then emits light of a wavelengthcharacteristic of the dopant, rather than the host.

Dopants can also serve to trap charge. For example, the LUMO levels ofthe host and dopant can be arranged such that the LUMO level of thedopant is lower than the LUMO level of the host, such that the dopantmolecule can act as an electron trap. Similarly, the HOMO levels of thehost and dopant can be arranged such that the HOMO level of the dopantis higher than the HOMO level of the host, such that the dopant moleculewould act as a hole trap. In addition, one or more dopants, referred toas transfer dopants, can be used to facilitate the transfer of energyfrom the host to the emissive dopant. For example, cascade doping can beused, which involves the non-radiative transfer of excitons from amolecule of the host through one or more transfer dopants to theemissive dopant. These intermediate transfers can be by Förstertransfer, Dexter transfer, hole trapping or electron trapping thateventually leads to the formation of an exciton on the transfer dopantor the emissive dopant, or by any other suitable mechanism.

Dopants can be present in the host material in quantities ranging, forexample, from about 0.1% to about 50%, from about 1% to about 20%, orfrom 1% to about 10% by weight. A level of about 1% by weight of dopingis preferred for emissive dopants in host material. Alternatively, insome embodiments, levels of dopant result in an average intermoleculardistance between dopant molecules of about the Förster radius of thedopant, such as, for example, from about 20 to about 40 Å, or from about25 to about 35 Å, or about 30 Å. Emissive dopants can include anycompound that is capable of photoemission. Emissive dopants includefluorescent organic dyes, such as laser dyes, as known and used in theart. Preferred emissive dopants include phosphorescent metal complexessuch as the Ir, Pt, and other heavy metal complexes disclosed in U.S.Pat. No. 6,303,238, United States Patent Application Publication No.2002/0034656, and 60/283,814, filed Apr. 13, 2001, each of which isherein incorporated by reference in its entirety.

In some embodiments, devices of the present invention comprise at leastone blocking layer. Blocking layers (BLs) function to confine holes,electrons, and/or excitons to specific regions of the light emittingdevices. For example, device efficiency can be increased when excitonsare confined to the EL and/or when holes and electrons are preventedfrom migrating out of the EL. Blocking layers can serve one or moreblocking functions. For example, a hole blocking layer can also serve asan exciton blocking layer. In some embodiments, the hole blocking layerdoes not simultaneously serve as an emissive layer in devices of thepresent invention. Although a blocking layer can include compounds thatare capable of emitting, emission can occur in a separate emissivelayer. Thus, in preferred embodiments, the blocking layer does notluminesce. Blocking layers can be thinner than carrier layers. Typicalblocking layers have thicknesses ranging from about 50 Å to about 1000Å, or from about 50 Å to about 750 Å, or from about 50 Å to about 500 Å.Additionally, blocking layers preferably comprise compounds other thanBAlq.

Hole blocking layers (HBLs) are typically comprised of materials thathave difficulty acquiring a hole. For example, hole blocking materialscan be relatively difficult to oxidize. In most instances, hole blockingmaterials are more difficult to oxidize than an adjacent layer fromtransporting holes. A material that is more difficult to oxidize thananother material typically possesses a lower HOMO energy level. Forexample, holes originating from the anode and migrating into an EL canbe effectively blocked from exiting the EL (on the cathode side) byplacing a blocking layer of material adjacent to the EL on the cathodeside of the device. The blocking layer preferably has a HOMO energylevel lower than the HOMO energy levels of the EL. Larger differences inHOMO energy levels correspond to better hole blocking ability. The HOMOof the materials of the blocking layer are preferably at least about 50,100, 200, 300, 400, 500 meV (milli-electronvolts) or more deeper thanthe HOMO level of an adjacent layer in which holes are to be confined.In some embodiments, the HOMO of the materials of the blocking layer isat least about 200 meV deeper than the HOMO level of an adjacent layerin which holes are to be confined.

In some devices of the invention, the layer in which holes are to beconfined can comprise more than one material, such as a host material(matrix) and a dopant. In this case, a HBL preferably has a HOMO energylevel that is lower (deeper) than the material of the adjacent layerwhich carries the majority of positive charge (i.e., the material withthe highest (shallowest) HOMO energy level). For example, an emissivelayer can comprise a host material having a deeper HOMO energy levelthan the dopant. In this case, the dopant acts as a trap for holes andcan be the principle hole transporter of the emissive layer. Thus, insuch embodiments, the HOMO energy of the dopant is considered whenselecting a hole blocking layer. Thus, in some embodiments, the HOMOenergy level of the HBL can be higher than the host material and lowerthan that of the dopant.

Hole blocking layers are also preferably good electron injectors.Accordingly, the LUMO energy level of the HBL is preferably close to theLUMO energy level of the layer in which holes are to be confined.Differences in LUMO energy levels between the two layers in someembodiments can be less than about 500 meV, 200 meV, 100 meV, 50 meV, oreven smaller. Hole blocking layers that are also good electron injectorstypically have smaller energy barriers to electron injection than forhole leakage. Accordingly, the difference between the LUMO energies ofthe HBL and the layer in which holes are to be confined (correspondingto an electron injection energy barrier) is smaller than the differencein their HOMO energies (i.e., hole blocking energy barrier).

Conversely, electron blocking layers (EBLs) are comprised of materialsthat have difficulty acquiring electrons (i.e., are relatively difficultto reduce). In the context of a light emitting device, EBLs arepreferably more difficult to reduce than the adjacent layer from whichelectrons migrate. A material that is more difficult to reduce thananother material generally has a higher LUMO energy level. As anexample, electrons originating from the cathode and migrating into an ELlayer can be blocked from exiting the EL (on the anode side) by placinga blocking layer adjacent to the anode side of the EL where the blockinglayer has a LUMO energy level higher than the LUMO energy level of theEL. Larger differences in LUMO energy levels correspond to betterelectron blocking ability. The LUMO of the materials of the blockinglayer are preferably at least about 50 meV, 100 meV, 200 meV, 300 meV,400 meV, 500 meV or more higher (shallower) than the LUMO level of anadjacent layer in which holes are to be confined. In some embodiments,the LUMO of the materials of the blocking layer can be at least about200 meV higher (shallower) than the LUMO level of an adjacent layer inwhich holes are to be confined.

In some embodiments, the layer in which electrons are to be confined cancomprise more than one material, such as a host material (matrix) and adopant. In this case, an EBL preferably has a LUMO energy level that ishigher than the material of the adjacent layer which carries themajority of negative charge (e.g., either the host or dopant having thelowest LUMO energy level). For example, an emissive layer can include ahost material having a deeper LUMO energy level than the dopant. In thiscase, the host can be the principle electron transporter of the emissivelayer. In such embodiments, the LUMO energy level of the EBL can behigher than the host material and lower than that of the dopant.Similarly, if the dopant served as the primary carrier of electrons,then the EBL preferably has a higher LUMO than the dopant.

Electron blocking layers are also preferably good hole injectors.Accordingly, the HOMO energy level of the EBL is preferably close to theHOMO energy level of the layer in which electrons are to be confined.Differences in HOMO energy levels between the two layers in someembodiments can be less than about 500 meV, 200 meV, 100 meV, 50 meV, oreven smaller. Electron blocking layers that are also good hole injectorstypically have smaller energy barriers to hole injection than forelectron leakage. Accordingly, the difference between the HOMO energiesof the EBL and the layer in which electrons are to be confined(corresponding to an hole injection energy barrier) is smaller than thedifference in their LUMO energies (i.e., electron blocking energybarrier).

Migration of excitons from the EL to other parts of the devices can beblocked with materials that have difficulty acquiring excitons. Transferof an exciton from one material to another may be prevented when thereceiving material has a wider (greater) optical gap than the excitondonating material. For example, excitons can be substantially confinedto the EL layer of a device by placing, adjacent to the EL layer, anexciton blocking layer having a wider optical gap than the materialscomprising the EL layer. Exciton blocking layers can also be placed oneither side of the EL. Exciton blocking layers can also serve as HBLs orEBLs, depending on the energy levels of the HOMO or LUMO of the excitonblocking material compared with those of adjacent layers (as discussedabove). Additionally, exciton blocking layers can be good electron orhole injectors when either the HOMO or LUMO energy level of the excitonblocking layer is close in energy to the respective HOMO or LUMO energylevel of an adjacent layer. For example, in devices having an excitonblocking layer and an emissive layer, the exciton blocking layer canhave a HOMO energy level that is less than about 500, 200, or 100 meVfrom the HOMO energy level of said emissive layer. Conversely, theexciton blocking layer can have a LUMO energy level that is less thanabout 500, 200, 100 meV from the LUMO energy level of said emissivelayer.

According to some embodiments of the present invention, blocking layerscan also comprise dopants. As an example, the blocking layer can becomprised of a wide band-gap matrix (host) material doped with a smallerband-gap dopant. Depending on the matrix and dopant combination, theeffective LUMO energy of the blocking layer can be lowered by thepresence of dopant, consequently improving the electron conduction andinjection properties of a hole blocking layer. Conversely, the effectiveHOMO energy of the blocking layer can be raised by the presence ofdopant, thereby improving hole injection properties. As an example, insome embodiments, HBLs comprise a wide band-gap matrix doped with asmaller band-gap material where the deep HOMO energy level of the matrixserves to prevent transport of holes and the relatively shallow LUMOlevel of the dopant favors electron injection. In some embodiments ofthe invention, the matrix can comprise a substantially conjugatedorganic molecule such as, for example, octaphenyl cyclooctatetraene(OPCOT), oligophenylenes such as hexaphenyl, and other similar materialshaving a wide band-gap. Suitable matrix band gap values can be at leastabout 3 eV, but can also be at least about 2.5 eV, 3.0 eV, 3.3 eV, 3.5eV or higher. Dopant is preferably a metal complex. Doping levels canrange from about 1% to about 50%, or more preferably from about 5% toabout 20%, or even more preferably from about 10 to about 15% by weight.An example of a suitable metal complex used as a dopant for blockinglayers is bis(2-(4,6-difluorophenyl)pyridyl-N,C2′)iridium(III)picolinate (FIrpic). An example of hole blocking layer comprising amatrix doped with a metal complex is OPCOT doped with 15% by weight ofFIrpic (OPCOT:FIrpic(15%)). For example, OPCOT:FIrpic can effectivelyconfine holes to an emissive layer comprising CBP doped with Ir(ppy)₃(tris(2-phenylpyridyl-N,C2′)iridium(III), Irppy) because the HOMO ofOPCOT is lower than the HOMO of Irppy and the LUMO of Flrpic is higherthan the LUMO of CBP.

Metal complexes used in the devices of the present invention include anymetal coordination complex comprising at least one metal atom and atleast one ligand. Metal complexes can be charged or uncharged; however,uncharged complexes are more amenable to the thin layer depositiontechniques used in OLED fabrication. Metal complexes are preferablystable to both one electron oxidation and one electron reductionprocesses. Redox-stable complexes can be identified, for example, bycyclic voltammetry (e.g., identification of reversible redox events).Additionally, such metal complexes often have low reorganizationalenergy barriers associated with oxidation and reduction. Accordingly,complexes having low reorganizational energy barriers show littlestructural difference between resting state, oxidized, and reducedstate. Metal complexes typically characterized as having lowreorganizational energy barriers include complexes having d⁰, d¹, d²,d³, d⁴, d⁵ and d⁶ electron configurations. For example, octahedralcomplexes having d³ or d⁶ metals typically generally have lowreorganizational energy barriers. Metal complexes in which redox eventsaffect predominantly non-bonding molecular orbitals (such as the t_(2g)set in octahedral transition metal complexes) generally have lowreorganizational energy barriers, since little structural change is seenin the ligand set upon oxidation or reduction. Reorganizational energyassociated with redox events can also be modulated by the ligand set.For example, multidentate ligands can structurally impose a certaincoordination geometry in metal complexes. Relatively rigid tridentate,tetradentate, hexadentate ligands, and the like can constraincoordination geometry such that redox events do not result insignificant structural reorganization. Additionally, metal complexesthat are coordinatively saturated, such as six-coordinate complexes,which are less likely to have significant structural change associatedwith oxidation or reduction, are also preferred. Four-coordinatecomplexes can also be suitable and can include both tetrahedral andsquare-planar complexes as well as others. Octahedral complexes are alsosuitable due to their propensity for forming glassy films. Metalcomplexes comprising aromatic ligands may help facilitate redoxprocesses, preferably in those instances where redox events are largelycentered on the ligand. Furthermore, metal complexes comprising heavymetals are preferred over those with lighter metals for their greaterthermal stability. For example, complexes comprising second and thirdrow transition metals are preferred.

Any metal atom, in any of its accessible oxidation states, is suitablein metal complexes, including main group, transition metals,lanthanides, actinides, alkaline earth, and alkali metals. Transitionmetals include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc,Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au, and Hg. Main groupmetals include Al, Ga, Ge, In, Sn, Sb, Tl, Pb, Bi, and Po. In someembodiments, metals having an atomic number greater than about 13, 36,or 54 are preferred.

Metal complexes can include any suitable ligand system. Suitable ligandscan be monodentate, bidentate, multidentate, π-bonding, organic,inorganic, charged, or uncharged. Further, ligands preferably compriseone or more heteroatoms through which the metal atom is coordinated,although organometallic compounds comprising coordinating carbon arealso suitable and also considered as metal complexes. Coordinatingheteroatoms of the ligands can include oxygen, nitrogen, sulphur,phosphorus, and the like. Nitrogen-containing ligands can includeamines, nitrenes, azide, diazenes, triazenes, nitric oxide,polypyrazolylborates, heterocycles such as 2,2′-bipyridine (bpy),1,10-phenanthroline, terpyridine (trpy), pyridazine, pyrimidine, purine,pyrazine, pyridine, 1,8-napthyridine, pyrazolate, imidazolate, andmacrocycles including those with and without a conjugated π system, andthe like. Phosphorus-containing ligands typically include phosphines andthe like. Oxygen-containing ligands include water, hydroxide, oxo,superoxide, peroxide, alkoxides, alcohols, aryloxides, ethers, ketones,esters, carboxylates, crown ethers, β-diketones, carbamate,dimethylsulfoxide, and oxo anions such as carbonate, nitrate, nitrite,sulfate, sulfite, phosphate, perchlorate, molybdate, tungstate, oxalate,and related groups. Sulfur-containing ligands can include hydrogensulfide, thiols, thiolates, sulfides, disulfides, thioether, sulfuroxides, dithiocarbamates, 1,2-dithiolenes, and the like. Ligandscomprising coordinating carbon atoms can include cyanide, carbondisulfide, alkyl, alkenes, alkynes, carbide, cyclopentadienide, and thelike. Halides can also serve as ligands. Metal complexes containingthese and other ligands are described in detail in Cotton and Wilkinson,Advanced Inorganic Chemistry, Fourth Ed., John Wiley & Sons, New York,1980, which is incorporated herein by reference in its entirety.Additional suitable ligands are described in Application Ser. Nos.60/283,814, filed Apr. 13, 2001 and United States Patent ApplicationPublication No. 2002/0034656, each of which is incorporated herein byreference in its entirety.

Ligands, especially neutral ligands, can be further derivatized with oneor more substituents, including anionic groups, to fully or partiallyneutralize any positive formal charge associated with the metal atoms ofthe metal complexes. Suitable anionic substituents can includecarbonate, nitrate, nitrite, sulfate, sulfite, phosphate, and the like.

Examples of suitable metal complexes for use in hole transporting layerscan include, inter alia, transition metal complexes having first,second, or third row transition metals, including, for example, Fe, Co,Ru, Pd, Os, and Ir. Further examples include coordinatively saturatedcomplexes and complexes that are six- or four-coordinate.

In some embodiments of the present invention, devices comprise a holetransporting layer comprising at least one metal complex of Formula I orII:

wherein:

M′ and M′″ are each, independently, a metal atom;

R₁₀, R₁₃, R₂₀, and R₂₁ are each, independently, N or C;

R₁₁ and R₁₂ are each, independently, N or C;

Ring systems A, B, G, K and L are each independently a mono-, di- ortricyclic fused aliphatic or aromatic ring system optionally containingup to 5 hetero atoms;

Z is C₁-C₆ alkyl, C₂-C₈ mono- or poly alkenyl, C₂-C₈ mono- or polyalkynyl, or a bond; and

Q is BH, N, or CH.

A particularly suitable compound of the above Formula I is Co(ppz)₃, thestructure of which is shown below.

Another particularly suitable compound of the above Formula II is irontrispyrazolylborate (FeTp′₂), the structure of which is shown below.

Metal complexes suitable for hole blocking layers can include, interalia, complexes of Os, Ir, Pt, and Au, including those described in U.S.Pat. No. 6,303,238, United States Patent Application Publication No.2002/0034656, and 60/283,814, filed Apr. 13, 2001, each of which isherein incorporated by reference in its entirety. An example of a metalcomplex suitable in hole blocking layers isbis(2-(4,6-difluorophenyl)pyridyl-N,C2′)iridium(III) picolinate(FIrpic), the structure of which is shown below.

Metal complexes suitable for electron blocking layers include those thatare relatively difficult to reduce (i.e., high LUMO energy level).Suitable metal complexes include metal complexes of the formula:

M is a metal atom;

X is N or CX′ where X′ is H, C₁-C₂₀ alkyl, C₂-C₄₀ mono- or poly alkenyl,C₂-C₄₀ mono- or poly alkynyl, C₃-C₈ cycloalkyl, aryl, heteroaryl,aralkyl, heteroaralkyl, or halo;

A is CH, CX′, N, P, P(═O), aryl or heteroaryl;

each R¹ and R² is, independently, H, C₁-C₂₀ alkyl, C₂-C₄₀ alkenyl,C₂-C₄₀ alkynyl, C₃-C₈ cycloalkyl, aryl, aralkyl, or halo; or

R¹ and R², together with the carbon atoms to which they are attached,link to form a fused C₃-C₈ cycloalkyl or aryl group;

R³ is H, C₁-C₂₀ alkyl, C₂-C₄₀ alkenyl, C₂-C₄₀ alkynyl, C₃-C₈ cycloalkyl,aryl, aralkyl, or halo; and

n is 1 to 5.

In some preferred embodiments, M is a trivalent metal such as Al or Ga.Variable A may preferably be CR³ or N. R¹ and R², in some embodiments,join to form a fused aromatic ring such as phenyl or pyridyl. Aparticularly suitable compound of the above formula isgallium(III)tris[2-(((pyrrole-2-yl)methylidene)amino)ethyl]amine(Ga(pma)₃) shown below.

Other suitable metal complexes may have the formula

wherein:

M is a metal atom;

is N or CX′ where X′ is H, C₁-C₂₀ alkyl, C₂-C₄₀ mono- or poly alkenyl,C₂-C₄₀ mono- or poly alkynyl, C₃-C₈ cycloalkyl, aryl, heteroaryl,aralkyl, heteroaralkyl, or halo;

each R¹ and R² is, independently, H, C₁-C₂₀ alkyl, C₂-C₄₀ alkenyl,C₂-C₄₀ alkynyl, C₃-C₈ cycloalkyl, aryl, aralkyl, or halo; or

R¹ and R², together with the carbon atoms to which they are attached,link to form a fused C₃-C₈ cycloalkyl or aryl group; and

R³ is H, C₁-C₂₀ alkyl, C₂-C₄₀ alkenyl, C₂-C₄₀ alkynyl, C₃-C₈ cycloalkyl,aryl, aralkyl, or halo.

As referred to throughout the present disclosure, alkyl groups includeoptionally substituted linear and branched aliphatic groups. Cycloalkylrefers to cyclic alkyl groups, including, for example, cyclohexyl andcyclopentyl, as well as heterocycloalkyl groups such as pyranyl, andfuranyl groups. Cycloalkyl groups may be optionally substituted. Alkenylgroups may be substituted or unsubstituted and comprise at least onecarbon-carbon double bond. Alkynyl groups may be substituted orunsubstituted and comprise at least one carbon-carbon triple bond. Arylgroups are aromatic and substituted aromatic groups having about 3 toabout 50 carbon atoms, including, for example, phenyl and naphthyl.Heteroaryl groups are aromatic or substituted aromatic groups havingfrom about 3 to about 50 carbon atoms and comprising at least oneheteroatom. Examples of heteroaryl groups include pyridyl and imidazolylgroups. Aralkyl groups can be substituted or unsubstituted and haveabout 3 to about 30 carbon atoms, and include, for example, benzyl.Heteroaralkyl include aralkyl groups comprising at least one heteroatom. Halo includes fluoro, chloro, bromo, and iodo. Substituted groupsmay contain one or more substituents. Suitable substituents may include,for example, H, C₁-C₂₀ alkyl, C₂-C₄₀ alkenyl, C₂-C₄₀ alkynyl, C₃-C₈cycloalkyl, C₃-C₈ heterocycloalkyl, aryl, heteroaryl, aralkyl,heteroaralkyl, halo, amino, azido, nitro, carboxyl, cyano, aldehyde,alkylcarbonyl, aminocarbonyl, hydroxyl, alkoxy, and the like.Substituents can also be electron-withdrawing groups andelectron-donating groups. As used herein, the term “hetero” is intendedto denote a non-carbon atom, for example N, O, or S.

Metal complexes suitable for exciton blocking layers include those thathave relatively wide optical gaps. Metal complexes suitable for thepreparation of hole blocking layers include high energy absorbers andemitters, such as for example, blue emitters. Preferred metal complexesinclude those in which the metal has a closed valence shell (no unpairedelectrons). As a result, many preferred metal complexes for preparingexciton blocking layers are colorless, since their optical gap energyfalls outside the visible range. Further, complexes having heavy metalsare preferred. For example, heavy metals of the second and third rowtransition series tend to have larger optical gaps due to a strongerligand field. Examples of suitable metal complexes for exciton blockinglayers include, inter alia, complexes of Os, Ir, Pt, and Au, such asthose described in U.S. Pat. No. 6,303,238, United States PatentApplication Publication No. 2002/0034656, and 60/283,814, filed Apr. 13,2001, each of which is herein incorporated by reference in its entirety.In some embodiments, metal complexes suitable for exciton blockinglayers include FIrpic, Ga(pma)₃, and related compounds.

According to some embodiments of the present invention, devices caninclude a hole transporting layer that includes at least one metalcomplex, and, in some embodiments, an electron blocking layer thatincludes, for example, organic material, a metal complex, or both.Suitable organic materials include any organic electron blockingmaterial known in the art such as, for example, triarylamines orbenzidenes. In some embodiments, the electron blocking layer residesbetween the HTL and EL. Accordingly, the electron blocking layer can beselected such that the HOMO energy level of the electron blocking layerfalls between the HOMO energy levels of the HTL and EL. In otherembodiments, the HOMO energy level of the electron blocking layer isclose to the HOMO energy level of the hole transporting layer. Forexample, the magnitude of the difference between the HOMO energy levelsof the electron blocking layer and the hole transporting layer can beabout 500, 200, 100, 50 meV or less. Examples of metal complexessuitable as electron blocking layers include complexes having Ga, In, Snor a transition metal such as a group 8, 9, or 10 transition metal.Other examples of suitable metal complexes include complexes havingmultidentate ligands. A particularly suitable metal complex is Ga(pma)₃.

While the metal complexes of the metal complex-containing HTLs describedherein can impart charge-carrying and/or blocking functions, the metalcomplexes also provide the additional advantage of increasing thethermal stability of devices of the invention. This is significantbecause OLEDs and similar light emitting devices can be subject toelevated temperatures for prolonged periods of time, and such exposureis thought to be a limiting factor in the lifetimes of such devices.Accordingly, devices of the invention containing for example, complexesof heavy metals such as second and third row transition metals andfourth and fifth row main group metals are envisaged to benefits interms of lifetime of the device.

The HOMO and LUMO energy levels for OLED materials, can be measured, orestimated, in several ways known in the art. The two common methods forestimating HOMO energy levels include solution electrochemistry, such ascyclic voltammetry, and ultraviolet photoelectron spectroscopy (UPS).Two methods for estimating LUMO levels include solution electrochemistryand inverse photoemission spectroscopy. As discussed above, alignment ofthe HOMO and LUMO energy levels of adjacent layers can control thepassage of holes and electrons between the two layers.

Cyclic voltammetry is one of the most common methods for determiningoxidation and reduction potentials of compounds. This technique is wellknown to those skilled in the art, and a simple description of thisprocess follows. A test compound is dissolved along with a highconcentration of electrolyte. Electrodes are inserted and the voltagescanned in either the positive or negative direction (depending onwhether an oxidation or reduction is being performed). The presence of aredox reaction is indicated by current flowing through the cell. Thevoltage scan is then reversed and the redox reaction is reversed. Thereference can be an external electrode, such as Ag/AgCl or SCE, or itcan be an internal one, such as ferrocene, which has a known oxidationpotential. The latter is often preferred for organic solvents, since thecommon reference electrodes are water based. A useful parameter that maycome from cyclic voltammetry is the carrier gap. If both the reductionand oxidation are reversible, one can determine the energy differencebetween the hole and the electron (i.e. taking an electron out of theHOMO versus putting one into the LUMO). This value can be used todetermine the LUMO energy from a well defined HOMO energy. Method fordetermining redox potentials and reversibility of redox events usingcyclic voltammetry is well known in the art.

UPS is an alternative technique for determining absolute bindingenergies in the solid state. Although solution electrochemistry istypically adequate for most compounds, and for giving relative redoxpotentials, the measurements taken in the solution phase can differ fromvalues in the solid phase. A preferred method of estimating HOMOenergies in the solid state is UPS. This is a photoelectric measurement,where the solid is irradiated with UV photons. The energy of the photonsare gradually increased until photogenerated electrons are observed. Theonset of ejected electrons gives the energy of the HOMO. The photons atthat energy have just enough energy to eject an electron from the top ofthe filled levels. UPS provides HOMO energy level values in eV relativeto vacuum which corresponds to the binding energy for the electron.

Inverse photoemission may be used to directly estimate LUMO energylevels. This technique involves pre-reducing the sample and then probingthe filled states to estimate the LUMO energies. More specifically, amaterial is injected with electrons which then decay into unoccupiedstates and emit light. By varying the energy of the incoming electronsand the angle of the incident beam, electronic structure of a materialcan be studied. Methods of measuring LUMO energy levels using inversephotoemission are well known to those skilled in the art.

Optical gap values can be determined from the intersection of thenormalized absorption and emission spectra. For molecules that have verylittle structural rearrangement in going from the ground state to theexcited, such that the gap between the absorption and emission 1_(max)values is rather small, the intersection energy is a good estimate ofthe optical gap (the 0-0 transition energy). Thus, the optical gaproughly corresponds to the HOMO-LUMO gap, and such estimation may beadequate for ideal systems. However, if the shift between the absorptionand emission maxima is large (Stokes shift) the optical gap can be moredifficult to determine. For example, if there is a structuralrearrangement in the excited state or the measured absorption does notrepresent the lowest energy excited state, then there can be asubstantial error. Thus, for the selection of potential exciton blockingmaterials, the edge of the absorption band of the material is preferablyused to obtain a value for its optical gap. In this way, device layerscomprising materials having absorption band energies higher than foradjacent layers may serve as effective exciton blocking layers. Forexample, if an exciton approaches a layer in a device having a higherenergy absorption edge than the material containing the exciton, theprobability that the exciton will be transferred into the higher energymaterial is low. For molecules emitting from triplet excited states, theabsorption edge is a preferred estimate for optical gap, since theintersystem crossing leads to a very large Stokes shift.

Light emitting devices of the present invention can be fabricated by avariety of techniques well known to those skilled in the art. Smallmolecule layers, including those comprised of neutral metal complexes,can be prepared by vacuum deposition, organic vapor phase deposition(OVPD), such as disclosed in U.S. Pat. No. 6,337,102, which isincorporated herein by reference in it its entirety, or solutionprocessing such as spin coating. Polymeric films can be deposited byspin coating and CVD. Layers of charged compounds, such as salts ofcharged metal complexes, can be prepared by solution methods such a spincoating or by an OVPD method such as disclosed in U.S. Pat. No.5,554,220, which is incorporated herein by reference in its entirety.Layer deposition generally, though not necessarily, proceeds in thedirection of the anode to the cathode, and the anode typically rests ona substrate. As such, methods of fabricating devices, involvingdepositing a blocking layer that comprises a metal complex onto apreexisting layer, are also encompassed by the present invention.Preexisting layers include any layer that is designed to be in contactwith the blocking layer. In some embodiments, the preexisting layer canbe an emissive layer or a HTL. Devices and techniques for theirfabrication are described throughout the literature and in, for example,U.S. Pat. Nos. 5,703,436; 5,986,401; 6,013,982; 6,097,147; and6,166,489. For devices from which light emission is directedsubstantially out of the bottom of the device (i.e., substrate side), atransparent anode material such as ITO may be used as the bottomelectron. Since the top electrode of such a device does not need to betransparent, such a top electrode, which is typically a cathode, may becomprised of a thick and reflective metal layer having a high electricalconductivity. In contrast, for transparent or top-emitting devices, atransparent cathode may be used such as disclosed in U.S. Pat. Nos.5,703,436 and 5,707,745, each of which is incorporated herein byreference in its entirety. Top-emitting devices may have an opaqueand/or reflective substrate, such that light is produced substantiallyout of the top of the device. Devices can also be fully transparent,emitting from both top and bottom.

Transparent cathodes, such as those used in top-emitting devicespreferably have optical transmission characteristics such that thedevice has an optical transmission of at least about 50%, although loweroptical transmissions can be used. In some embodiments, devices includetransparent cathodes having optical characteristics that permit thedevices to have optical transmissions of at least about 70%, 85%, ormore. Transparent cathodes, such as those described in U.S. Pat. Nos.5,703,436 and 5,707,745, typically comprise a thin layer of metal suchas Mg:Ag with a thickness, for example, that is less than about 100 Å.The Mg:Ag layer can be coated with a transparent,electrically-conductive, sputter-deposited, ITO layer. Such cathodes areoften referred to as compound cathodes or as TOLED (transparent-OLED)cathodes. The thickness of the Mg:Ag and ITO layers in compound cathodesmay each be adjusted to produce the desired combination of both highoptical transmission and high electrical conductivity, for example, anelectrical conductivity as reflected by an overall cathode resistivityof about 30 to 100 ohms per square. However, even though such arelatively low resistivity can be acceptable for certain types ofapplications, such a resistivity can still be somewhat too high forpassive matrix array OLED pixels in which the current that powers eachpixel needs to be conducted across the entire array through the narrowstrips of the compound cathode.

The present invention further includes methods of facilitating holetransport in a light emitting device, wherein the light emitting devicepreferably comprises a hole transporting layer and an emissive layer,and wherein the hole transporting layer comprises at least one metalcomplex. In accordance with preferred embodiments of the methods, thedevice is designed such that the emissive layer has a higher HOMO energylevel than the HOMO energy level of the hole transporting layer. In someembodiments, the methods involve placing an electron blocking layerbetween the HTL and EL, where the HOMO energy level of the electronblocking layer has a HOMO energy level between that of the HTL and EL.

The present invention further includes methods of transporting holes ina hole transporting layer of a light emitting device, where said holetransporting layer comprises at least one metal complex, comprisingapplying a voltage across a device of this structure.

Structures of light emitting devices are often referred to by asequential listing of layer materials separated by slashes. For example,a device having an anode layer adjacent to a hole transporting which isadjacent to an emissive layer which is adjacent to an electron blockinglayer which is adjacent to a cathode layer can be written asanode/HTL/EL/ETL/cathode. As such, devices of the present invention caninclude the substructures HTL/EL/HBL, HTL/EBL/EL, HTL/EBL/ETL, anothers. Some preferred structures of the present invention includeanode/HTL/EL/HBL/ETL/cathode and anode/HTL/EBL/EL/ETL/cathode. Otherembodiments include devices with substructures HTL/EL or HTL/EBL/ELwhere each of the EL, HTL, and EBL comprise at least one metal complexor where none of the EL, HTL, or EBL comprise solely organic molecules.Further embodiments include a device, having a plurality of layers,devoid of a layer that is composed solely of organic molecules or adevice having a plurality of layers where each of the layers contains atleast one metal complex.

Light emitting devices of the present invention can be used in a pixelfor a display. Virtually any type of display can incorporate the presentdevices. Displays can include computer monitors, televisions, personaldigital assistants, printers, instrument panels, bill boards, and thelike. In particular, the present devices can be used in heads-updisplays because they can be substantially transparent when not in use.

As those skilled in the art will appreciate, numerous changes andmodifications can be made to the preferred embodiments of the inventionwithout departing from the spirit of the invention. It is intended thatall such variations fall within the scope of the invention.

Throughout this specification various groupings are employed toconveniently describe constituent variables of compounds and groups ofvarious related moieties. It is specifically intended that eachoccurrence of such groups throughout this specification include everypossible subcombination of the members of the groups, including theindividual members thereof.

It is intended that each of the patents, applications, and printedpublications mentioned in this patent document be hereby incorporated byreference in their entirety.

EXAMPLES Example 1 Synthesis of Tris(1-phenylpyrazole-C²,N′) cobalt(III) (Co(ppz)₃)

To a solution of 1-phenylpyrazole (1.0 equiv. Mol, 6.93 mmol), in THF (3ml), was added ethylmagnesium bromide (1.1 equiv. Mol, 7.6 mmol). Thereaction mixture was refluxed for three hours under argon atmosphere,and then cooled in a dry ice/acetone bath. A solution of cobalt (II)bromide (0.5 equiv mol, 3.47 mmol) in THF (8 ml) was then added slowlyto the Grignard. Immediately the reaction mixture turned black. The bathwas removed and the mixture was stirred two days.

The reaction mixture was transferred to a separatory funnel containingaqueous NH₄Cl (10 g/liter, 75 ml) and CH₂Cl₂ (75 ml) and shook. Theresulting thick emulsion was passed through a course fritted funnel,which aided the separation of the two layers. The organic layer wasisolated and the aqueous layer was extracted twice more with CH₂Cl₂ (75ml). The CH₂Cl₂ solutions were combined, dried over MgSO₄, filtered andconcentrated under reduced pressure. Addition of hexanes to theblack/yellow concentrate caused the product to precipitate as a darkyellow solid. ¹H NMR analysis indicated that the crude product was amixture of facial and meridonal isomers, as well as a small amount ofimpurities. An attempt was made to separate the two isomers andimpurities by column chromatography, using silica gel and 1:1CH₂Cl₂:toluene eluent. Only the facial isomer exited the column, whereasthe meridonal isomer stuck to the silica and soon after turned purple incolor.

Example 2 Synthesis of Tris(1-(4-tolylphenyl)pyrazole-C²,N′) cobalt(III) (CoMPPZ)

Ligand synthesis (4-tolylphenylpyrazole): A 250 ml flask was chargedwith a magnetic stirrer bar, 4-tolylboronic acid (16 mmol, 2.0 equiv.),pyrazole (8 mmol, 1.0 equiv.), anhydrous cupric acetate (12 mmol, 1.5mmol), 6 g activated 4 Å molecular sieves, pyridine (16 mmol, 2.0equiv.), and 96 ml dichloromethane. The reaction was stirred under airat ambient temperature in a loosely capped flask for 2 days. Thereaction mixture was filtered through Celite, washed with water andpurified by silica gel chromatography (eluent:ethyl acetate:hexane=1:7)

Complex synthesis: To a solution of 4-tolyllpyrazole (1.0 equiv. Mol,6.93 mmol), in THF (3 ml), was added ethylmagnesium bromide (1.1 equiv.Mol, 7.6 mmol). The reaction mixture was refluxed for three hours underargon atmosphere, and then cooled in a dry ice/acetone bath. A solutionof cobalt (II) bromide (0.5 equiv mol, 3.47 mmol) in THF (8 ml) was thenadded slowly to the Grignard. Immediately the reaction mixture turnedblack. The bath was removed and the mixture was stirred two days.

The reaction mixture was transferred to a separatory funnel containingaqueous NH₄Cl (10 g/liter, 75 ml) and CH₂Cl₂ (75 ml) and shook. Theresulting thick emulsion was passed through a course fritted funnel,which aided the separation of the two layers. The organic layer wasisolated and the aqueous layer was extracted twice more with CH₂Cl₂ (75ml). The CH₂Cl₂ solutions were combined, dried over MgSO₄, filtered andconcentrated under reduced pressure. Addition of hexanes to theblack/yellow concentrate caused the product to precipitate as a darkyellow solid. ¹H NMR analysis indicated that the crude product was amixture of facial and meridonal isomers, as well as a small amount ofimpurities. An attempt was made to separate the two isomers andimpurities by column chromatography, using silica gel and 1:1CH₂Cl₂:toluene eluent. Only the facial isomer exited the column, whereasthe meridonal isomer stuck to the silica and soon after turned purple incolor.

Example 3 Synthesis of Tris(1-(4-methoxyphenyl)pyrazole-C²,N′) cobalt(III) (CoMOPPZ)

Ligand synthesis (4-methoxyphenylpyrazole): A 250 ml flask was chargedwith a magnetic stirrer bar, 4-methoxyboronic acid (16 mmol, 2.0equiv.), pyrazole (8 mmol, 1.0 equiv.), anhydrous cupric acetate (12mmol, 1.5 mmol), 6 g activated 4 Å molecular sieves, pyridine (16 mmol,2.0 equiv.), and 96 ml dichloromethane. The reaction was stirred underair at ambient temperature in a loosely capped flask for 2 days. Thereaction mixture was filtered through Celite, washed with water andpurified by silica gel chromatography (eluent:ethyl acetate:hexane=1:7)

Complex synthesis: To a solution of 4-methoxypyrazole (1.0 equiv. Mol,6.93 mmol), in THF (3 ml), was added ethylmagnesium bromide (1.1 equiv.Mol, 7.6 mmol). The reaction mixture was refluxed for three hours underargon atmosphere, and then cooled in a dry ice/acetone bath. A solutionof cobalt (II) bromide (0.5 equiv mol, 3.47 mmol) in THF (8 ml) was thenadded slowly to the Grignard. Immediately the reaction mixture turnedblack. The bath was removed and the mixture was stirred two days.

The reaction mixture was transferred to a separatory funnel containingaqueous NH₄Cl (10 g/liter, 75 ml) and CH₂Cl₂ (75 ml) and shook. Theresulting thick emulsion was passed through a course fritted funnel,which aided the separation of the two layers. The organic layer wasisolated and the aqueous layer was extracted twice more with CH₂Cl₂ (75ml). The CH₂Cl₂ solutions were combined, dried over MgSO₄, filtered andconcentrated under reduced pressure. Addition of hexanes to theblack/yellow concentrate caused the product to precipitate as a darkyellow solid. ¹H NMR analysis indicated that the crude product was amixture of facial and meridonal isomers, as well as a small amount ofimpurities. An attempt was made to separate the two isomers andimpurities by column chromatography, using silica gel and 1:1CH₂Cl₂:toluene eluent. Only the facial isomer exited the column, whereasthe meridonal isomer stuck to the silica and soon after turned purple incolor.

Example 4 Synthesis of Tris(1-(4,5-difluorophenyl)pyrazole-C²,N′) cobalt(III) (CodFPPZ)

Ligand synthesis (4,5-difluorophenylpyrazole): A 250 ml flask wascharged with a magnetic stirrer bar, 4,5-difluoroboronic acid (16 mmol,2.0 equiv.), pyrazole (8 mmol, 1.0 equiv.), anhydrous cupric acetate (12mmol, 1.5 mmol), 6 g activated 4 Å molecular sieves, pyridine (16 mmol,2.0 equiv.), and 96 ml dichloromethane. The reaction was stirred underair at ambient temperature in a loosely capped flask for 2 days. Thereaction mixture was filtered through Celite, washed with water andpurified by silica gel chromatography (eluent:ethyl acetate:hexane=1:7)

Complex synthesis: To a solution of 4,5-difluoropyrazole (1.0 equiv.Mol, 6.93 mmol), in THF (3 ml), was added ethylmagnesium bromide (1.1equiv. Mol, 7.6 mmol). The reaction mixture was refluxed for three hoursunder argon atmosphere, and then cooled in a dry ice/acetone bath. Asolution of cobalt (II) bromide (0.5 equiv mol, 3.47 mmol) in THF (8 ml)was then added slowly to the Grignard. Immediately the reaction mixtureturned black. The bath was removed and the mixture was stirred two days.

The reaction mixture was transferred to a separatory funnel containingaqueous NH₄Cl (10 g/liter, 75 ml) and CH₂Cl₂ (75 ml) and shook. Theresulting thick emulsion was passed through a course fritted funnel,which aided the separation of the two layers. The organic layer wasisolated and the aqueous layer was extracted twice more with CH₂Cl₂ (75ml). The CH₂Cl₂ solutions were combined, dried over MgSO₄, filtered andconcentrated under reduced pressure. Addition of hexanes to theblack/yellow concentrate caused the product to precipitate as a darkyellow solid. ¹H NMR analysis indicated that the crude product was amixture of facial and meridonal isomers, as well as a small amount ofimpurities. An attempt was made to separate the two isomers andimpurities by column chromatography, using silica gel and 1:1CH₂Cl₂:toluene eluent. Only the facial isomer exited the column, whereasthe meridonal isomer stuck to the silica and soon after turned purple incolor.

Example 5 Synthesis of Tris(1-(4-fluorophenyl)pyrazole-C²,N′) cobalt(III) (CoFPPZ)

Ligand synthesis (4-fluorophenylpyrazole): A 250 ml flask was chargedwith a magnetic stirrer bar, 4-fluoroboronic acid (16 mmol, 2.0 equiv.),pyrazole (8 mmol, 1.0 equiv.), anhydrous cupric acetate (12 mmol, 1.5mmol), 6 g activated 4 Å molecular sieves, pyridine (16 mmol, 2.0equiv.), and 96 ml dichloromethane. The reaction was stirred under airat ambient temperature in a loosely capped flask for 2 days. Thereaction mixture was filtered through Celite, washed with water andpurified by silica gel chromatography (eluent:ethyl acetate:hexane=1:7)

Complex synthesis: To a solution of 4-fluoropyrazole (1.0 equiv. Mol,6.93 mmol), in THF (3 ml), was added ethylmagnesium bromide (1.1 equiv.Mol, 7.6 mmol). The reaction mixture was refluxed for three hours underargon atmosphere, and then cooled in a dry ice/acetone bath. A solutionof cobalt (II) bromide (0.5 equiv mol, 3.47 mmol) in THF (8 ml) was thenadded slowly to the Grignard. Immediately the reaction mixture turnedblack. The bath was removed and the mixture was stirred two days.

The reaction mixture was transferred to a separatory funnel containingaqueous NH₄Cl (10 g/liter, 75 ml) and CH₂Cl₂ (75 ml) and shook. Theresulting thick emulsion was passed through a course fritted funnel,which aided the separation of the two layers. The organic layer wasisolated and the aqueous layer was extracted twice more with CH₂Cl₂ (75ml). The CH₂Cl₂ solutions were combined, dried over MgSO₄, filtered andconcentrated under reduced pressure. Addition of hexanes to theblack/yellow concentrate caused the product to precipitate as a darkyellow solid. ¹H NMR analysis indicated that the crude product was amixture of facial and meridonal isomers, as well as a small amount ofimpurities. An attempt was made to separate the two isomers andimpurities by column chromatography, using silica gel and 1:1CH₂Cl₂:toluene eluent. Only the facial isomer exited the column, whereasthe meridonal isomer stuck to the silica and soon after turned purple incolor.

Example 6 Synthesis of Tris(1-(4-tert-butylphenyl)pyrazole-C²,N′) cobalt(III) (CoBPPZ)

Ligand synthesis (4-tert-phenylpyrazole): A 250 ml flask was chargedwith a magnetic stirrer bar, 4-tert-boronic acid (16 mmol, 2.0 equiv.),pyrazole (8 mmol, 1.0 equiv.), anhydrous cupric acetate (12 mmol, 1.5mmol), 6 g activated 4 Å molecular sieves, pyridine (16 mmol, 2.0equiv.), and 96 ml dichloromethane. The reaction was stirred under airat ambient temperature in a loosely capped flask for 2 days. Thereaction mixture was filtered through Celite, washed with water andpurified by silica gel chromatography (eluent:ethyl acetate:hexane=1:7)

Complex synthesis: To a solution of 4-tert-pyrazole (1.0 equiv. Mol,6.93 mmol), in THF (3 ml), was added ethylmagnesium bromide (1.1 equiv.Mol, 7.6 mmol). The reaction mixture was refluxed for three hours underargon atmosphere, and then cooled in a dry ice/acetone bath. A solutionof cobalt (II) bromide (0.5 equiv mol, 3.47 mmol) in THF (8 ml) was thenadded slowly to the Grignard. Immediately the reaction mixture turnedblack. The bath was removed and the mixture was stirred two days.

The reaction mixture was transferred to a separatory funnel containingaqueous NH₄Cl (10 g/liter, 75 ml) and CH₂Cl₂ (75 ml) and shook. Theresulting thick emulsion was passed through a course fritted funnel,which aided the separation of the two layers. The organic layer wasisolated and the aqueous layer was extracted twice more with CH₂Cl₂ (75ml). The CH₂Cl₂ solutions were combined, dried over MgSO₄, filtered andconcentrated under reduced pressure. Addition of hexanes to theblack/yellow concentrate caused the product to precipitate as a darkyellow solid. ¹H NMR analysis indicated that the crude product was amixture of facial and meridonal isomers, as well as a small amount ofimpurities. An attempt was made to separate the two isomers andimpurities by column chromatography, using silica gel and 1:1CH₂Cl₂:toluene eluent. Only the facial isomer exited the column, whereasthe meridonal isomer stuck to the silica and soon after turned purple incolor.

Example 7 Synthesis ofGa(III)tris[2-(((pyrrole-2-yl)methylidene)amino)ethyl]amine (Ga(pma)₃)

The ligand [(((pyrrole-2-yl)methylidene)amino)ethyl]amine was preparedby adding a methanolic solution of pyrrole-2-carboxaldehyde (1.430 g, 15mmol, 100 ml) to a methanolic solution of tris(2-aminoethyl)amine (0.720g, 5 mmol, 10 ml). The resulting yellow solution was stirred at roomtemperature for 30 min. A methanolic solution of gallium(III) nitratehydrate (1.280 g, 5 mmol, 150 ml) was added to the ligand solution andstirred at room temperature for 30 min. The solution was filtered andleft to stand at ambient temperature until crystallization occurred. Thecrude material was then sublimed at 235° C.

Example 8 Devices of the Present Invention

Functional OLEDs were prepared with the structure ITO/HTL/Alq₃/MgAgwhere Co(ppz)₃ was used as the HTL. These devices had lower efficiencythan for comparable devices in which the HTL was composed oftriarylamines. Higher efficiencies were achieved for these devices whena thin layer of NPD was inset between the Co(ppz)₃ and Alq₃ layers. TheHOMO levels for both NPD and Co(ppz)₃ are at comparable energies (5.5 eVfor NPD and 5.4 eV for Co(ppz)₃). See FIGS. 13-15.

Example 9 Devices According to Embodiments of the Present Invention

Functional OLEDs were prepared with a Co(ppz)₃ hole transporting layer,a Ga(pma)₃ electron blocking layer and an Alq₃ emissive layer. The turnon voltage was roughly 3.5 V (voltage at which externalbrightness=Cd/m²), and the peak efficiency was greater than 1.2%. SeeFIGS. 7-10.

Example 10 Devices According to Embodiments of the Present Invention

Functional OLEDs were prepared with a Co(ppz)₃/Ga(pma)₃ holetransporting layer and a doped CBP emissive layer. An analogous devicewas also prepared with an NPD hole transporting layer. Overall, theCo(ppz)₃/(Ga(pma)₃ device performed better than the device with the NPDlayer. The low voltage currents are lower and the spectra show no blueemission (below 400 nm) for Co(ppz)₃/(Ga(pma)₃. The turn-on voltages forthe Co(Ppz)₃/(Ga(pma)₃ based device was slightly higher than for theNPD-based device and the quantum efficiency was lower. Both of theseparameters are expected to improve with device optimization. See FIGS.11-12.

1. A light emitting device comprising a hole transporting layer, whereinsaid hole transporting layer comprises at least one organometalliccomplex, having at least one coordinating carbon, the organometalliccomplex comprising at least one multi-dentate ligand.
 2. The lightemitting device of claim 1, wherein said hole transporting layerconsists essentially of said organometallic complex.
 3. The lightemitting device of claim 1, wherein said hole transporting layercomprises an organic matrix doped with said organometallic complex. 4.The light emitting device of claim 1, wherein said organometalliccomplex is coordinatively saturated.
 5. The light emitting device ofclaim 1, wherein said organometallic complex has a coordination numberof six.
 6. The light emitting device of claim 1, wherein saidorganometallic complex has a coordination number of four.
 7. The lightemitting device of claim 1, wherein said metal of said organometalliccomplex is a transition metal.
 8. The light emitting device of claim 7,wherein said transition metal is a first row transition metal.
 9. Thelight emitting device of claim 7, wherein said transition metal is asecond or third row transition metal.
 10. The light emitting device ofclaim 1, wherein the metal of said organometallic complex is Fe, Co, Ru,Pd, Os or Ir.
 11. The light emitting device of claim 1, wherein themetal of said organometallic complex is Fe or Co.
 12. The light emittingdevice of claim 1, wherein the metal of said organometallic complex isFe.
 13. The light emitting device of claim 1, wherein the metal of saidorganometallic complex is Co.
 14. The light emitting device of claim 1,wherein said organometallic complex has one of the formulas I or II:

wherein: M′ and M′″ are each, independently, a metal atom; R₁₀, R₁₃,R₂₀, and R₂₁ are each, independently, N or CH; R₁₁ and R₁₂ are each,independently, N, CH, O or S; Ring systems A, B, G, K and L are eachindependently a mono-, di- or tricyclic fused aliphatic or aromatic ringsystem optionally containing up to 5 hetero atoms; Z is C₁-C₆ alkyl,C₂-C₈ mono- or poly alkenyl, C₂-C₈ mono- or poly alkynyl, or a bond; andQ is BH, N, or CH.
 15. The light emitting device of claim 1, wherein theligand is bidentate.
 16. The light emitting device of claim 1, whereinthe ligand is tridentate.
 17. The light emitting device of claim 1,wherein the ligand is tetradentate.
 18. The light emitting device ofclaim 1, wherein the ligand is hexadentate.
 19. The light emittingdevice of claim 14, wherein M′ is Co.
 20. The light emitting device ofclaim 15, wherein M′ is Co.